A primary concern in the design of a control system for a mechanical press is safety. A mechanical press typically has a reciprocating ram moving alternately towards and away from a die or mold piece. In most applications, such as in metal stamping, a powerful force is used to drive the ram towards the die. If the ram is inadvertently driven, a danger potential is created. Thus, it is important that the control system of a mechanical press does not fail in a manner causing the ram to be driven towards the die.
In the past, relay control circuits have been generally preferred over solid state control systems for mechanical presses. A relay physically breaks an electrical circuit with a reliable and reassuring airspace, whereas a solid state control system breaks or opens an electrical circuit by greatly increasing the resistivity of a simiconductor device. This functional distinction between relays and solid state control devices made it impractical to substitute solid state devices in a relay circuit.
The primary safety concerns with solid state control systems may be placed in two categories. First, there exists a possibility that the final output semiconductor device or devices that activate the ram may fail and inadvertently activate the ram. Second, there exists a possibility that an input solid state device or an intermediate solid state device may fail creating a spurious signal that would cause the final output solid state device or devices to activate the ram. Thus, a need has arisen for a solid state control system having a control circuit designed to substantially reduce the possibility that a failure of a final output device, an intermediary device, or an input device would cause the circuit to activate the ram.
Many prior mechanical presses have used inch buttons when the ram is stopped on an upstroke. In many mechanical presses, a number of operating stations are each provided with a pair of run buttons, but only one set of inch buttons are usually provided. To operate the ram of the mechanical press, all of the run buttons must be pressed in concert. However, in a typical mechanical press control system, the run buttons may be used to start the operation of the mechanical press only when the ram is in position for a downstroke. If the ram is stopped on the upstroke, generally, it was necessary to use the one set of inch buttons to bring the ram back to the top of its stroke in preparation for another downstroke.
If the run buttons on such mechanical presses are being used to operate the press, each operator must generally be pressing his buttons before the press will operate. However, if the inch buttons are used, only one operator is controlling the press, and a possibility exists that another operator will inadvertently be injured by the moving ram. Thus, a need has arisen for a control system for a mechanical press in which the run buttons may be used to bring the ram back to the top of the ram stroke after the ram has been stopped during the ram upstroke.
In many conventional control systems, the press is placed in a continuous mode of operation by pressing a continuous preset button and then pressing the run buttons within a selected time period after the continuous preset button is depressed. The run buttons must then be held in the depressed position until the ram reaches a selected position in the ram cycle. At this point, the run buttons may be released and the ram will continue to operate in the continuous mode until a stop signal is generated by some other means.
If the run buttons were not depressed within the selected time period after the continuous preset button was depressed, the ram would not operate. In such event, the operator could be confused as to the reason that the ram was not operating. Typically, such prior systems provide no indication of what fault prevented the operation of the ram. Also, when the condition preset button was depressed, the run buttons were depressed, and the ram began operation, typically, there is no clear indication to the operator of the point in time when the run buttons may be released without stopping the ram. Thus, a possibility exists that the operator will inadvertently release the run buttons too soon or will hold the run buttons in a depressed position for a needlessly long period of time. Thus, a need has arisen for indicators to transmit information to the operator to facilitate his operation of the press.